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Sleep cycles: What are the stages of sleep?

brain with a clock

After a long day, there is possibly nothing better than a good night’s sleep.

Sleep is vital for our health and well-being, allowing our bodies to rest, recover, repair, and grow. But, whilst you’re figuring out how many hours you need (or can get), there’s one more thing you need to think about, and that is your sleep quality.

This is where your sleep cycle comes in. But what is a sleep cycle? And why is it important?

Keep reading to find out all you need to know about sleep cycles, how to calculate them and more on each stage of sleep.

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What is a sleep cycle?

When you are asleep, your brain cycles through four stages – this is known as a sleep cycle. No night is the same, and your brain will likely cycle through several sleep cycles in one night, up to as many as 6!1

On average, one sleep cycle will last around 90 minutes, but this can vary throughout the night.

For instance, your first sleep cycle of the night is usually between 70 and 100 minutes, with each cycle getting longer as the night goes on.2

Your sleep cycle will likely be different to someone else’s and even different each night, as it depends on various factors like age and recent sleep patterns. But understanding and even calculating sleep cycles can help us figure out the key to a good night’s sleep.

What are the different stages of sleep?

There are four stages of sleep that each have distinct patterns and characteristics.

Each of the four types of sleep play a crucial role in supporting your brain and body’s function, with some stages also being critical in repairing your body and getting you ready for the day ahead.

These stages of sleep fall into two categories: rapid eye movement (REM) and non-rapid eye movement (NREM). Your sleep cycle will repeat throughout the night, with each REM stage increasing in length and depth.3

NREM Sleep

NREM sleep comprises of 3 stages and involves deeper sleep in its later stages. In fact, the higher the NREM stage, the more difficult it is to wake someone up.

Stage 1: N1

The first sleep stage is NREM N1. It is the shortest sleep stage and is often described as the “dozing off” stage.

Generally, this sleep stage will last between 1 - 5 minutes, and your body will start to wind down and relax. You might also experience twitching as your muscles begin to settle into sleep.4

If you are undisturbed during N1 sleep, you’ll move into the next stage quickly, but it can be pretty easy to wake someone up in N1.

 

Stage 2: N2

After N1, your body moves into the second sleep stage, N2.

You’ll typically spend most of your time asleep in N2. It can last between 10 and 25 minutes, getting progressively longer in your nightly sleep cycles.

During N2:

  • your body becomes even more relaxed
  • your heart rate lowers
  • your breathing rate slows
  • your temperature drops
  • you become less aware of your surroundings4

In this NREM sleep stage, your brain will produce rhythmic, quick brain waves known as sleep spindles. These are thought to be the result of your brain gathering and processing new memories from the previous day.5

Stage N2 helps your body to prepare for sleep stages N3 and REM, which are deeper stages of sleep where your brain and body reset for the next day.6

 

Stage 3: N3

The third stage of sleep is N3, also known as deep sleep, which is vital for helping you feel refreshed the next day.3

When you’re in stage N3, your brain activity slows to deeper brain waves, known as delta waves. This brain activity signals that you are in a deep sleep state, and any stimuli, like noises or activity, might fail to wake you up.

During N3, your muscles become completely relaxed, your blood pressure drops, your breathing slows, and you move into the deepest stage of your sleep.

Scientists believe that it is during N3 that your body starts to grow and recover, consolidate memories, and even encourage creativity and support your memory.7,8

You’ll spend the most time in N3 during the first half of your night’s sleep, with this stage lasting for around 20 to 40 minutes. But, as the night goes on, you will spend more time in REM sleep instead.

 

REM: What is REM sleep?

You might have heard about this stage of your sleep cycle, but what is it, and when is REM sleep in the stages of your sleep cycle?

REM sleep is the fourth, final stage of your sleep cycle and the second stage of deep sleep.

When you’re in REM sleep, your brain activity closely resembles its activity whilst you are awake. But your muscles become immobilised to prevent you from acting out what happens in your dreams.3

It is thought that REM sleep occurs about 90 minutes after falling asleep.

During REM sleep:

  • your body completely relaxes, and your muscles become temporarily immobilised
  • your brain becomes more active
  • your breathing rate increases and becomes more irregular
  • your eyes move rapidly
  • you experience more dreams3

Like N3, your brain gathers and consolidates memories during REM sleep. But it is thought that it is during REM sleep that your brain processes emotions and emotional memories.9

REM sleep is crucial, it allows your body to repair and build cells, secrete hormones, and promote muscle and bone growth. This is also an excellent time for your body to fight off illness and infection to keep you feeling healthy.3

What happens when your sleep cycle is interrupted?

Whenever you struggle with falling asleep or staying asleep throughout the night, your sleep cycle will be affected in turn.

This is called “interrupted sleep”, where your sleep is not continuous throughout the night. When you wake up during the night, this can cut short your sleep stage and interrupt your sleep cycle as a result.

Many things can interrupt your sleep cycle, including10:

  • Your age: your sleep becomes lighter as you age, so you will naturally wake up more often.
  • Sleep disorders: conditions like obstructive sleep apnoea, restless leg syndrome, and delayed sleep-wake phase syndrome can keep you up at night.
  • Your mental health: sleep can become more complex with things like anxiety and depression.
  • Nocturia: if you have nocturia, you’ll wake up frequently at night to go to the toilet.
  • Chronic pain: conditions that cause chronic pain can make it difficult to fall and stay asleep.
  • Your lifestyle: smoking, drinking alcohol and caffeine can all affect your ability to fall asleep.11

When your sleep cycle is interrupted, you might have trouble focusing, being creative, making decisions, solving problems, remembering things, and controlling your emotions.12

How many hours of sleep should I get a day?

How much sleep you need each night varies throughout your life.

But it is thought that the average sleep time is:

  • Between 7 - 9 hours for adults
  • Between 9 - 13 hours for children
  • Between 12 - 17 hours for toddlers and babies13

Can you get into a sleep cycle during a nap?

If you feel tired or didn’t get a good night’s sleep, then a nap during the day could help.

But, to feel refreshed and more alert after a nap, you’ll probably need to sleep for around 90 minutes. This is the average length of your sleep cycle, so napping for 90 minutes will allow your body to go through an entire sleep cycle (including REM sleep) and be more beneficial for you.

Napping for less time is perfectly okay, too, though, as it helps you to process memories and remember facts, faces and places. But you might feel a bit groggy afterwards.14

How to calculate your sleep cycle?

If you’re always thinking, “what time should I go to bed?” every evening, you could try calculating your sleep cycle.

You can use a basic formula or sleep time calculator to figure out how much sleep you want in a night, using the time you must get up in the morning.

The average sleep cycle is around 90 minutes long, and you will have approximately 5 sleep cycles in one night - that is roughly 450 minutes or 7.5 hours of sleep.15

So, to figure out the best time for you to go to sleep, work back 7.5 hours from your wake-up time, and you’ll find the best time for you to go to sleep.

For example, if you need to wake up at 7 am, your ideal bedtime is 11:30 pm. This will allow your body to cycle through each stage of sleep and help you feel more refreshed in the morning!

But everybody is different, so it’s best to use this calculation as a guideline for figuring out your own unique sleep cycle.

Calculating your sleep cycle can also help reduce your sleep debt. This is the difference between the amount of sleep you need and the amount you get.

So, if you need 8 hours but only get 6, you can use your sleep time calculator to figure out the additional hours you need to eliminate your sleep debt.16

The bottom line

It turns out there’s more to sleep than we think!

Each sleep stage has important effects on our brains and bodies. As your body goes through the four stages of sleep, from stages 1, 2, 3 and REM sleep, it moves through different processes that affect your breathing, heart rate, cells, muscles, bones and even temperature.

At the same time, your brain is busy gathering, forming, and processing memories.

Your sleep cycle follows a pattern, but it can be interrupted due to certain habits, your age, and some health conditions.

Sleep is incredibly important, and over time, a lack of sleep can have some negative effects on your body.

Learn more about how you can get a good night’s sleep in our guide to sleeping better!

The advice in this article is for information only and should not replace medical care. Please check with your GP or healthcare professional before trying any supplements, treatments or remedies. Food supplements must not be used as a substitute for a varied and balanced diet and a healthy lifestyle. 

Last updated: 13 January 2023

Sources

  1. https://www.ncbi.nlm.nih.gov/books/NBK526132/
  2. http://healthysleep.med.harvard.edu/healthy/science/what/sleep-patterns-rem-nrem
  3. https://www.ninds.nih.gov/health-information/public-education/brain-basics/brain-basics-understanding-sleep
  4. https://elifesciences.org/articles/18894
  5. https://dx.doi.org/10.1523%2FJNEUROSCI.2604-11.2011
  6. https://www.sleepfoundation.org/how-sleep-works/what-happens-when-you-sleep
  7. https://pubmed.ncbi.nlm.nih.gov/21377416/
  8. https://doi.org/10.3389/fpsyg.2015.00622
  9. https://doi.org/10.1038/s41598-020-74169-8
  10. https://www.sleepfoundation.org/sleep-deprivation/interrupted-sleep
  11. https://www.cdc.gov/sleep/data_statistics.html
  12. https://dx.doi.org/10.2147/NSS.S134864
  13. https://www.nhs.uk/conditions/insomnia/
  14. https://sites.bu.edu/ombs/2013/10/18/the-perfect-nap-with-the-most-brain-benefits/
  15. https://thesleepdoctor.com/sleep-calculator/
  16. https://www.sleepfoundation.org/how-sleep-works/sleep-debt-and-catch-up-sleep
 

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